1. Field of the Invention
This invention relates to processing a hydrocarbon feedstock, and more particularly to steam reforming a hydrocarbon feedstock.
2. Description of the Prior Art
The production of hydrogen by the process of steam reforming is well known in the art. Briefly, a mixture of hydrocarbon fuel and steam, usually in vapor form, is passed through a vessel containing a steam reforming catalyst. Typically, the vessel is a catalyst filled tube disposed within a furnace. Heat from the furnace drives the endothermic steam reforming reaction. It is possible, with current technology, to convert all of the available hydrocarbons to essentially hydrogen, carbon monoxide, and carbon dioxide; however, disadvantages of prior art techniques for achieving very high conversion rates are one or more of the following: (1) higher furnace temperatures, (2) larger or more reactors, and (3) burning additional fuel in the furnace.
One typical technique for increasing the conversion rate is to pass the reaction products from a primary catalyst bed into and through a second catalyst bed while adding air to the second bed and burning some of the hydrogen already produced; this increases the temperature of the reaction products as they flow through the second bed to yield additional hydrogen. This technique is shown in U.S. Pat. No. 2,700,598 to Odell. Although there may be some advantages to this approach, clearly all the available hydrogen in the process fuel cannot become a product of the reactor since some of it is burned within the reactor.
Another technique is shown in Faatz, Jr. U.S. Pat. No. 2,813,779. In Faatz, Jr. a primary steam reforming reactor is disposed within a furnace. Reaction products from the primary reactor are preheated in the furnace to a high temperature and are then passed through a second catalyst bed external of the furnace wherein additional conversion of hydrocarbon takes place in an adiabatic reaction using the sensible heat in the preheated primary reaction products. Faatz, Jr. claims that by his invention unreacted hydrocarbons are reduced to concentrations less than 0.3 mole percent (1) without increasing the number of tubes in the furnace, (2) without increasing the temperature in the furnace, and (3) without increasing the amount of catalyst employed. For this to be true the Faatz, Jr. primary reaction apparatus must be relatively inefficient. The teachings of Faatz, Jr. cannot be utilized with very efficient primary reaction apparatus, since extracting heat from the furnace to preheat the reaction products would be inconsistent with maximizing efficiency within the primary reaction apparatus. Perhaps the reason Faatz, Jr. claims he can extract heat energy from the furnace to preheat the primary reaction products without having any negative effect on the primary reaction process is because the furnace contains a large amount of what otherwise would be waste heat (i.e., heat which is available for use in the primary reaction tubes but which cannot be used therein due to inefficiencies of the system). Furthermore, Faatz, Jr. does not lend itself to being compact since the second catalyst bed is outside of the furnace.